1. Field of the Invention
The present invention relates to an organic electrolyte battery which can improve the safety under extremely severe conditions such as nailing and breakage of batteries.
2. Description of the Related Art
Realization of practical use of an electric vehicle and the storage of electrical energy of night generation have been required from the viewpoint of environmental problems, such as air pollution, the increase of a carbon dioxide, etc., as well as the effective use of energy. Accordingly, an excellent secondary battery having characteristics such as high efficiency, high power density, high energy density, light weight, etc., has been demanded. From these points of view, a secondary battery using an organic electrolyte solution which has an; energy density several time larger than conventional batteries using an aqueous electrolyte, has been expected to be put to practical use.
As a positive active material of an organic electrolyte secondary battery, various materials have been examined, e.g., a titanium disulfide, a lithium-cobalt oxide, a lithium-nickel oxide, a lithium-manganese oxide, a vanadium oxide, a molybdenum sulfide, a molybdenum oxide, etc., and these active materials are retained on foils, e.g., aluminum, tantalum, stainless steel, etc., which are used as a positive plate (Unexamined Japanese Patent Publication (kokai) Nos. Hei. 5-290854 and Hei. 4-121962).
Various materials have been heretofore investigated as a negative active material of an organic electrolyte secondary battery and a lithium-based negative plate has attracted public attention in view of expectations of high energy density. Lithium metals, lithium alloys and carbons, oxides and sulfides capable of occluding and releasing lithium ions have been examined, and these active materials are retained on foils, e.g., stainless steel (Unexamined Japanese Patent Publication No. Hei. 5-29021), brass, phosphor bronze, and aluminum bronze (Unexamined Japanese Patent Publication No. Hei. 5-36401), and copper (Japanese Patent Publication No. Hei. 7-192724), etc., which are used as a negative plate.
Organic electrolyte solutions now used comprise an aprotic organic solvent containing a metal salt as an electrolyte dissolved therein. For example, with respect to lithium salts, LiClO.sub.4, LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiCF.sub.3 SO.sub.3, etc., are dissolved in propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, .gamma.-butyrolactone, sulfone, sulfolan, dioxolan, 2-methyltetrahydrofuran, dimethyl carbonate, diethyl carbonate, etc.
One example of a cylindrical organic electrolyte secondary battery is shown in FIG. 1. In FIG. 1, reference numeral 1 represents a case serving also as a negative plate terminal; 2, a positive electrode; 3, a negative electrode; 4, a separator; 5, a positive lead; 7, a positive terminal; 8, a safety valve; 9, a PTC element; 10, a gasket; and 11, an insulating plate. The positive electrode 2, the separator 4 and the negative electrode 3 are rolled up and housed in the case 1. The electrolyte solution is not shown in FIG. 1. FIG. 1 shows a cylindrical lithium secondary battery but a prismatic battery has substantially the analogous constitution.
Organic electrolyte batteries have high energy density and many of organic electrolyte solutions used therein are combustible. If a large current flows due to a short circuit or an erroneous use of a battery, abnormal heat is generated and there are possibilities of firing and battery bursting.
For preventing such phenomena, a fuse and a PTC element have been used in an electric circuit to break the flowing of a large current or a safety valve has been used to release the internal pressure of a battery container. Further, against an abnormally large current in a battery such as short circuits and nail penetration tests, a special separator having the function of operating at a specific temperature to reduce the discharge current of a battery, which is called a shutdown function, has been used.
Nail penetration tests are test methods prescribed in "safety evaluation standard guideline of lithium secondary batteries (SBAG1101)", and in "the guideline for safety evaluation of lithium batteries for cameras", Battery Association of Japan. The tests are supposing the severest internal short circuits due to the breakage of batteries.
FIG. 2 is a schematic view showing a nail penetration test. A metal bar having an acute tip (e.g., a nail) is stuck through an organic electrolyte secondary battery 20 held horizontally using a hydraulic driving means or a motor-driving means, and the safety of the battery is evaluated by the degrees of the generation of heat and the internal pressure increase of the battery resulting from the internal short circuit.
FIG. 3 is a schematic view showing the state of the internal short circuit caused by the nail penetration test. By the penetration of a nail 21 through a positive electrode 2, a negative electrode 3 and a separator 4, a positive substrate 2a having a positive active material 2b on both sides thereof and a negative substrate 3a having a negative active material 3b on both sides thereof are electrically connected via a nail 21 to thereby result in the state of a short circuit. Small-sized organic electrolyte secondary batteries now on the market are constructed such that safety can be ensured even under such a condition.
However, as to large-sized organic electrolyte secondary batteries, in particular, for use in electric vehicles, if an internal short circuit happens, an abnormally large current flows locally which cannot bear comparison with small-sized batteries, therefore, an organic electrolyte battery having a higher reliable safety structure than those heretofore in use has been demanded.